Chickens recognise human beauty, starlings can be pessimistic and elephants grieve for their fallen comrades: these are the perhaps surprising claims of Dr Jonathan Balcombe, an independent animal behaviour research scientist. In a new book, he argues that a flood of studies of species ranging from minnows to monkeys adds up to a revolution in our understanding of the way other animals experience the world. "Just 30 years ago, it was scientific heresy to ascribe such emotions as delight, boredom or joy to a non-human," he writes in Second Nature: The Inner Lives of Animals (Palgrave Macmillan). "[But now] researchers have found that there is more thought and feeling in animals than humans have ever imagined."

For Balcombe, who was born in the UK and now lives on the east coast of the States, this new understanding leads to radical conclusions.

So the idea that chickens have an aesthetic sense, for instance... we should pay attention to this because it seems to have profound implications for the way in which we treat animals.

For much of the 20th century, it was taboo to ask questions about what animals think and what they feel. That's changed; now we have a spate of studies of phenomena that show that animals are, in all the important ways, sentient in the manner that we are. They may not lead the same sorts of lives that we have, but they feel pleasure and pain just as intensely. They have just as acute emotional experiences as we do – there are studies showing that there are real inner lives to these animals.

What does this say about our relationship to animals? The paradox is that as our knowledge of animals increases, our treatment of them falls further behind because we still live according to a might-makes-right strategy, which is the kind of thinking that justified colonialism and slavery. Unfortunately, our treatment of animals remains pretty much medieval.

Could you clarify this point about sentience?

Sentience is the capacity for feeling things, usually pleasures and pains – but it's a very broad concept, ranging from grief to optimism, from positive to negative feelings. The key is that it's the bedrock of ethics: humans are moral beings, what the philosophers would call moral agents, who are able to make moral decisions, and sentience is what makes those moral systems. It's what makes a human life important – it's what makes murder, ultimately, a terrible crime. It's because you've deprived the individual of a future life. Given that animals are acutely sentient in the important ways – I'm not talking about building computers or reading books – we need to expand our moral circle to include them.

In a study titled "Chickens prefer beautiful humans", human faces were photographed and digitised, so they could be presented to undergraduates, who then rated them according to attractiveness. The male faces were rated by female students and vice versa. They came up with a gradation of the most and least attractive. Then chickens were presented with the same faces and strikingly, the chickens' preferences in binary choices, for whatever reason, showed a 98 per cent overlap with the humans' ratings.

It doesn't necessarily mean that the chickens found those faces more attractive – though that's what the authors seem to suggest. What meaning that has in a chicken's world I don't know. But what it does say to me is that they're very perceptive about cues and those perceptions are very similar to ours in terms of aesthetics.

In your book you write: "Why should chickens find us beautiful at all? Today in the US we kill and cause suffering to more chickens than there are human beings on the entire planet." Surely there's a danger of taking individual studies and over-interpreting them,reading too much into them. This particular study had six chickens in it: to go from the preferences they exhibited to an idea about beauty, isn't that a bit of a leap?

In the time it took you to ask that question, about 3,500 chickens were slaughtered in the US, so that gives an idea of the scale of the killing of these particular animals. Some biologists would call them the most successful animals on earth because there are so many of them now, but I would call them the least successful because the vast majority lead such short, miserable lives. If you look at it from a population outcome, it's a very different outcome than if you look at it as individuals, but it's individuals who are sentient.

But just because chickens rate humans in the same way as humans do in this particular study that doesn't mean their sentience is the same as ours, and I certainly wouldn't claim that. Although I wouldn't necessarily claim that they're less sensitive to the pain of a broken wing than we are to the pain of a broken arm: that can be debated. That is accessible to science. But what the study does say – and this is just one study of many – is that chickens have lives that matter. And this gets to the heart of some of the cognitive and emotional studies that I'm trying to bring to light: animals don't merely live in the moment. They have lives, moods and dispositions. They really have a welfare in the richest sense of the term. That means we have to look hard and reconsider the current relationship we have with them.

People tend to see faces in clouds or hear voices in draughty buildings; we're very good at anthropomorphising our surroundings. Whether what we now believe that animals experience is really the same thing as human emotions is a very open question, isn't it?

Here's an example of an emotional study that's based on rigorous science. It's a study of the chacma baboon. These particular populations have been studied in Botswana for 30 years by the same scientists, so you have long-term observational and experimental studies with animals in their own milieu. It's known that for women who lose an infant, it's a terrible, traumatic, sad event. It's known that they grieve, naturally, and that grief is reflected physiologically by an increase of glucocorticoid hormones in their blood for a month or more. These baboons show a similar pattern of behaviour; if a baboon mother in the Okavango Delta in Botswana loses her baby, the scientists are able to measure the hormone levels in her blood in a very non-invasive way. They don't have to take a blood sample, they simply keep an eye out and when the female defecates they shout: "Oh, Lucy just shat, go get that!" and they analyse the faeces.

This shows that their glucocorticoid levels also go up for about a month. And the levels of their closest associates, of the female baboons which they relate to and have friendships with, also go up: there's an osmotic effect. Again in parallel with humans; humans rally round, support each other, increase our social networks. Baboons show a similar pattern. Females who have lost an infant will groom more during that month and receive more grooming. It's thought to be a form of therapy to help overcome the grief that presumably they're experiencing.

This is the challenge with emotions: they're private feelings – and that's why science did neglect these questions for a long time.

There are all sorts of nasty things that go on with predators catching prey, male lions killing the young when they join a new pride. Do animals have any kind of moral responsibility?

Absolutely. One of the frontiers of science is this study of virtue in animals: increasingly it's coming to light that animals have a moral awareness, or a moral consideration about how they behave. This is particularly the case with social animals, who've evolved to live in groups. Living in groups is full of compromise, you give and take and you want to sustain good relationships with others or you may be an outcast, and that's not in your self-interest – so one can make genetic arguments for the evolution of virtue and moral behaviour – certainly we manifest it in many ways.

A recent study of dogs shows they have what's called inequity aversion – that is to say a fairness awareness. If you you have two dogs sitting next to each other, and you offer to shake their paws, and you only reward one of the dogs when they both shake paws, after about 10 or 12 trials the one who's not getting the treat will refuse to shake paws. Whereas the other one will continue happily to shake paws. In control experiments, when there's only one dog not getting any treats, he will continue shaking the paw much longer. So it's not just fatigue or frustration, there's an awareness that this is not correct – "He's getting rewarded for what he's doing and I'm not getting rewarded for doing the same thing."

Do plants have an intrinsic value?

Albert Schweitzer advocated an idea of "reverence for life". He coined that phrase in 1915 and I love it. It speaks to plants because plants are also living organisms. We could quibble about whether or not they're sentient, although most scientists would say they're not. But even if they aren't sentient, we should respect them and that relates to the broader issues of our relationship to animals.

People often ask me as a vegetarian – a vegan in fact – "What about plants? If you're a vegetarian, you're consuming plants!" But if you're a meat-eater you're indirectly consuming many more plants, because you're higher on the food chain and cows have to eat plants to make muscle. Being a vegetarian is a more plant-friendly way of life anyway. I believe in respecting all life – I don't want to see trees uprooted any more than I want to see cows slaughtered, but there is a moral difference because cows are sentient and a plant is not.